Is there some obvious detail that proves the account with 500K btc that got hacked and flash-sold, didn't belong to MtGox?

Someone who had recently hacked into MtGox, stolen user lists etc, might well have noticed such an account. Having already demonstrated hostility to MtGox, and maybe figuring there was no way they could outright steal the 500K btc and get away with it, couldn't they have decided to make MtGox 'shoot themselves in the head' by flash-selling their own bitcoin hoard?

Would certainly explain MtGox's quick response to rollback. They want _their_ money back.

(I'm new here, sorry if that scenario is unworkable due to some detail I'm not aware of.)